Ice control about marine structures

ABSTRACT

Ice is controlled about a marine structure after a continuous ice cover has been formed by applying a layer of solid insulating material sufficiently thick to balance heat float when the ice reaches the desired thickness to the surface of the ice.

United States Patent Hemstock ICE CONTROL ABOUT MARINE STRUCTURES [72] Inventor: Russell A. Helmtock, Calgary,

Canada [73] Assignee: Esso Production Research Company [22] Filed: March 3, 1971 [21] Appl. No.: 120,565

[52] US. Cl. ..62/66, 61/54, 62/259, 1 14/40 [51] Int. Cl. ..F25c l/02, E02d 5/22 [58] Field of Search...6l/l, 46 X, 54 X, 369; 62/259, 62/260, 66, 340; 114/40 X [451 Nov. 14, 1972 ABSTRACT Ice is controlled about a marine structure after a continuous ice cover has been formed by applying a layer of solid insulating material sufficiently thick to balance heat float when the ice reaches the desired thickness to the surface of the ice.

[56] References Cited 12 Clains, 4 Drawing Figures UNITED STATES PATENTS 3,170,299 2/1965 Clarke .6l/54 2( ll '5 1 I4 PATENT-BMW 14 1972 SHEET 1 BF 2 FIG. 2

1 1V VEIV'TOR.

RUSSELL Av HEMSTOCK BYW a Q L AT TORNEY ICE CONTROL ABOUT MARINE STRUCTURES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the control of ice in the vicinity of piles, piers, ships and other partially submerged objects normally exposed to severe icing conditions. I

2. Description of the Prior Art In many arctic areas, water near the coast is covered by shore-fast ice during much of the year. The ice thickness increases rapidly during the early fall, reaches a maximum in late winter, and diminishes in the spring. Experience has shown that this ice moves in response to wind, current and thermal forces and that such movement may amount to several feet during a season. The forces exerted by the moving ice on pilings and similar members extending through the ice cover may be quite large and hence the design and maintenance of piers, offshore platforms and similar structures present severe problems. Sleeves and similar devices which can be installed around the legs of such structures to prevent the freezing of water adjacent the legs and thus avoid lifting of the structures by the ice have been proposed but such methods do not eliminate difficulties due to horizontal movement of the shorefast ice cover.

SUMMARY OF THE INVENTION This invention provides a method for controlling the thickness of the ice cover around a marine structure so that ice moving against the structure never becomes thick enough to cause significant damage. In accordance with the invention, a layer of foamed plastic, sawdust, wood chips, snow or similar insulating material of predetermined thickness is applied to the surface of the ice around the marine structure after a complete ice cover has been formed and the surface is safe to work on. This layer should be sufficiently thick to balance heat flow through the ice after the desired ice thickness has been attained and should extend over an area sufficiently large to prevent contact of the unprotected ice with the structure as the ice cover moves. By thus controlling the thickness of the ice that contacts the structure, the forces exerted against it can be kept sufficiently low to avoid significant damage. Studies indicate that this method permits the use of substantially conventional structures in shore-fast ice areas and will allow year round operations in areas which are now accessible for only very limited periods during the year. It is substantially less expensive than methods requiring melting of the ice.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a vertical elevation showing an offshore oil production platform which has been protected against damage due to the movement of shore-fast ice in accordance with the invention;

FIG. 2 is a top view of a platform of the type shown in FIG. 1 which illustrates the distribution of insulating material in an area where the direction of movement of the shore-fast ice is unpredictable;

FIG. 3 is a top view similar to that of FIG. 2 which illustrates the distribution of insulating material in an area where movement of the shore-fast ice is predictable;

FIG. 4 illustrates the use of insulating material for controlling the thickness of shore-fast ice along a route over which a tanker or similar vessel must pass.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The offshore oil production platform shown in FIG. 1 of the drawing is a monopod type structure having an upright leg 11 on which a deck 12 is mounted. The wells on the structure extend downwardly through the leg into the earth beneath the water. The production equipment normally mounted on the platform is not shown. It should be understood that the method of the invention is not restricted to this particular type of platform and instead is equally applicable to any offshore structure which extends upwardly through shore-fast ice.

The water surrounding the platform of FIG. 1 is covered by a layer of shore-fast ice 13. A snow fence or similar barrier 14 has been erected on the surface of the ice around the leg of the platform by drilling holes into the ice, inserting stakes or similar supporting members, and allowing them to freeze in place. Bales of hay that can be wet down and frozen into place or other barriers may be used if desired. The use of a barrier is not always essential and may in some cases be dispensed with.

The direction in which the shore-fast ice will move during the weather season is often unpredictable and hence the barrier will generally be arranged in a circular pattern around the leg of the platform as shown in FIG. 2 of the drawing. There are some areas, however, where the direction of the wind and current forces are such that the ice moves back and forth along a relatively straight line and in those cases a long narrow pattern such as that shown in FIG. 3 of the drawing may be used in lieu of a circular pattern. In either case, the distance separating the barrier from the leg of the platform should be somewhat greater than the maximum distance the ice is expected to move during the winter season.

The enclosed space within barrier 14 has been filled with a layer of insulating material 15. Any of a variety of fibrous, granular or sheet materials having suitably low coefficients of thermal conductivity may be employed. Suitable materials include, for example, expanded silica, asbestos fibers, corrugated cardboard, cotton batting, granulated or sheet cork, diatomaceous earth, kapok, mineral wool, sawdust, slag wool, foamed-in-place plastics, crumpled paper, wood chips or shavings, snow, and expanded synthetic resins such asthe expanded phenol-formaldehydes, urea-formaldehydes, polyurethanes, polyethylenes, cellulose acetates, and the like. Certain of these materials, of course, have better properties than others and are therefore preferred. In general, materials which have thermal conductivities on the order of about 0.02 to 0.03 BTU/hr./ft /F/ft., possess sufficient strength to avoid excessive packing, and are unaffected by the presence of water are most effective.

A particularly effective class of expanded materials for purposes of the invention are the cellular polyurethanes. These can be prepared by the reaction of the dicarboxylic acid ester of a polyfunctional alcohol containing free hydroxyl groups, an adipic acid ester of glycerol, for example, with a dior polyisocyanate such as toluene diisocyanate. The dior polyisocyanate reacts in part with the hydroxyl groups on the ester to form polyurethane bonds. Due to the increase in temperature as the exothermic reaction takes place, the carboxyl groups react with isocyanate group to form cross links and liberate carbon dioxide. A tough spongy mass which turns into a very hard rigid foam is formed. A thermal blowing agent such as trichloromonofluoromethane, Unicel or Porofor may be included to increase the production of gas and thus obtain a lower density material. Surface active agents, fillers and other additives can also be included if desired. This method and other techniques which may be used to produce rigid polyurethane foams, including the use of free flowing powders which melt, expand and cure when heated, have been described in detail in the literature and will be familiar to those skilled in the art.

Many of the insulating materials suitable for purposes of the invention have very low densities and must be confined to prevent them from being blown away. It is therefor generally preferred to place the insulating material in waterproof bags and pack these into the confined space between the barrier and leg of the platform. Plastic bags used to ship and store the material are generally suitable. This protects the insulating material against wind and water, facilitates its placement around the leg of the platform, and permits its recovery at the end of the season. In lieu of this, the ice between the barrier and platform can be covered with a sheet of polyethylene or similar waterproof plastic material, loose insulating material can be placed on top of this sheet, and the insulating material can then be covered with a second sheet of plastic or similar material to protect it. Other placement methods can also be used.

As pointed out earlier, the insulating material is applied to the surface of the ice after a continuous ice cover sufficiently thick to work on has been formed. An ice layer from about 4 to about 6 inches thick is generally preferred. This will normally permit safe installation of the insulating material and will not cause any appreciable damage due to ice movement against the structure.

The thickness of the layer of insulating material applied to the ice will depend in part upon the climatic conditions in the area and the particular insulating material selected. The layer applied should be sufficient to prevent the formation of thick ice that might damage the structure but should not be thick enough to cause any significant thawing of the ice. The thickness required can be approximated by subtracting the thickness of the ice at the time the material is applied from the normal ice cover thickness under midwinter conditions and then calculating the amount of insulating material that will be necessary to provide the same insulation afforded by that quantity of ice. A more accurate method, however, is to calculate the thickness of insulating material necessary to balance heat flow through the ice under various temperature conditions. This will result in a series of thickness values that can be used as a basis for applying insulation in stages as the weather becomes colder and the temperature decreases. in some areas, the climatic conditions are such that it is preferable to apply the insulation in stages and in other areas they are such that it is normally satisfactory to apply all of the insulation at one time.

The presence of the insulating material on the ice surrounding the platform retards heat flow from the water through the ice to the colder air above it. As shown in FIG. 1 of the drawing, the ice beneath the insulating material therefore remains relatively thin. The surrounding ice, on the other hand, increases in thickness as the winter season progresses. Movement of the ice cover will result in crushing of the protected ice against the leg of the platform without any appreciable damage to the structure. If the platform were surrounded by ice of normal thickness, severe damage might well occur. As the protected ice is crushed against the platform a portion of the layer of insulating material will be destroyed. This material can be replaced as soon as the water refreezes about the structure and the new ice is safe to work on. The insulating material can be removed from the surface of the ice after the winter season has passed and can be stored for reuse the following season.

The method of the invention is not restricted to the protection of fixed structures such as that shown in FIG. 1 and instead can also be used for the protection of ships and other floating vessels and to facilitate the movement of such vessels in arctic port or terminal areas. in areas where a tanker must dock at infrequent intervals, for example, it may be advantageous to apply sufficient insulation 20 to the ice 21 along the approach 22 to such an area as illustrated in FIG. 4 to prevent the formation of very thick ice which would otherwise preclude movement of the vessel 23. Similarly, in some cases it may be advisable to apply insulation to the ice covering channels adjacent refineries and similar installations to facilitate the movement of fire boats and other emergency vehicles if necessary. Still other applications of the method will suggest themselves to those skilled in the art.

I claim:

1. A method for controlling the thickness of ice in a selected area at a marine location which comprises erecting a barrier about said area after an initial ice cover has been formed and applying a layer of thermal insulating material of predetermined thickness to the surface of the ice enclosed by said barrier, said layer being sufficiently thick to retard heat flow through the ice without causing significant thawing.

2. A method for reducing damage to a marine structure due to the movement of shore-fast ice which comprises applying a layer of thermal insulating material of predetermined thickness to the surface of the ice after an initial ice cover sufficiently thick to work on has been formed, said layer being sufficiently thick to retard heat flow through the ice without causing significant thawing and said layer being applied over an area extending from said structure a distance greater than the maximum anticipated movement of the ice.

3. A method as defined by claim 2 wherein said thermal insulating material is a synthetic plastic.

4. A method as defined by claim 2 wherein said thermal insulating material is enclosed by a barrier erected on the surface of said ice.

5. A method as defined by claim 2 wherein said thermal insulating material is applied over a circular pattern surrounding said structure.

6. A method as defined by claim 2 wherein the thickness of said layer of thermal insulating material is increased as the weather becomes colder.

7. A method as defined by claim 2 wherein said thermal insulating material is a finely divided solid having a thermal conductivity on the order of about 0.02 to 0.03 BTU/hr./ft F./ft.

8. A method as defined by claim 2 wherein said thermal insulating material comprises wood chips.

9. A method for facilitating the movement of a floating vessel along a predetermined route covered by shorefast ice which comprises applying a layer of solid insulating material of predetermined thickness to the surface of said ice along said route after an ice cover sufficiently thick to work on has been formed, said layer of insulating material being sufiicient to substantially balance heat flow through the ice after the ice reaches a predetermined thickness but insufficient to cause significant thawing of the ice.

10. A method as defined by claim 9 wherein said insulating material is a foamed plastic.

11. A method as defined by claim 9 wherein said insulating material is contained in waterproof bags.

12. A method as defined by claim 9 wherein said insulating material is placed on top of a sheet of waterproof plastic and is covered by a second sheet of plastic material. 

2. A method for reducing damage to a marine structure due to the movement of shore-fast ice which comprises applying a layer of thermal insulating material of predetermined thickness to the surface of the ice after an initial ice cover sufficiently thick to work on has been formed, said layer being sufficiently thick to retard heat flow through the ice without causing significant thawing and said layer being applied over an area extending from said structure a distance greater than the maximum anticipated movement of the ice.
 3. A method as defined by claim 2 wherein said thermal insulating material is a synthetic plastic.
 4. A method as defined by claim 2 wherein said thermal insulating material is enclosed by a barrier erected on the surface of said ice.
 5. A method as defined by claim 2 wherein said thermal insulating material is applied over a circular pattern surrounding said structure.
 6. A method as defined by claim 2 wherein the thickness of said layer of thermal insulating material is increased as the weather becomes colder.
 7. A method as defined by claim 2 wherein said thermal insulating material is a finely divided solid having a thermal conductivity on the order of about 0.02 to 0.03 BTU/hr./ft2/*F./ft.
 8. A method as defined by claim 2 wherein said thermal insulating material comprises wood chips.
 9. A method for facilitating the movement of a floating vessel along a predetermined route covered by shore-fast ice which comprises applying a layer of solid insulating material of predetermined thickness to the surface of said ice along said route after an ice cover sufficiently thick to work on has been formed, said layer of insulating material being sufficient to substantially balance heat flow through the ice after the ice reaches a predetermined thickness but insufficient to cause significant thawing of the ice.
 10. A method as defined by claim 9 wherein said insulating material is a foamed plastic.
 11. A method as defined by claim 9 wherein said insulating material is contained in waterproof bags.
 12. A method as defined by claim 9 wherein said insulating material is placed on top of a sheet of waterproof plastic and is covered by a second sheet of plastic material. 